Semiconductor amplifier and electrode structures therefor



Nov. 11, 1 952 BARDEEN AL 2,617,865

SEMICONDUCTOR AMPLIFIER AND ELECTRODE STRUCTURES THEREFOR Original Filed June 17, 1948 .3 Sheets-Sheet 1 INPUT FIG. 2

'Ec FIG.

4' 3 7 a? w a E55 I 7 RL .BARDLEN g 1% H. BRA7'7I4/N A T TOR/VE V 1952 J.-BARDEEN ETAL 2,6 7,

SEMICONDUCTOR AMPLIFIER AND ELECTRODE STRUCTURES THEREFOR "Original F-iled June 17, 1948 s Sheets-Sheet 2 INVENTORSWH. BRATTA/N AT TORNEY Nov. 11, 1952 BARDEEN ETAL 2,617,865

SEMICONDUCTOR AMPLIFIER AND ELECTRODE STRUCTURES THEREFOR Original Filed June 17, 1948 '3 Sheets-Sheet 3 BARR/ER N TYPE FIG/5 R E: t

DEPTH A fly l r I z y l T JBARDEEN 'WHBRATTA/N a'mfrn/ A 7' TORNE V Patented Nov. 11, 1952 SEIVIICONDUCTOR AMPLIFIER AND ELEC- TRODE STRUCTURESTHEREFOR John Bardeen, Summit and Walter H. Brattain, Morristown, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a

corporation of New York Original application June 17, 1948, Serial No. 39,466, now Patent No. 2,524,035, dated October 3, 1950. Divided and'this application September 15, 1949, Serial No. 115,838

1'7 Claims. (01. 175-366) This application is a division of application Serial No. 33,466,'filed June 1'7, 1948, issued October 3, 1950 as Patent 2,524,035.

This invention relates to a novel method of and means for translating electrical variations for such purposes as amplification, wave generation, and the like.

The principal object of the invention is to amplify or otherwise translate electric signals or variation-s by use of compact, simple, and rugged apparatus of novel type.

Another object is to provide a circuit element for use as an amplifier or the like which does not require a heated thermionic cathode for its operation, and which therefore is immediately operative when turned on. ,A related object is to provide such a circuit element which requires no evacuated or gas-filled envelope.

Attempts have been made'in the past to convert solid rectifiers utilizing selenium, copper sulfide, or other semi-conductive materials into amplifiers by the direct expedient of embedding a grid-like electrode in a dielectric layer disposed between thecathode and the anode of the rectifier. The grid is supposed, by exerting an electric force at the surface of the cathode, to modify its emission and so alter the cathode-anode current. As a practical matter it is impossible to embed a grid in a layer which is so thick as to insulate the grid from' the other electrodes and yet so thin as to permit'current to flow between them. It has also been proposedto pass a current from end to'end of a strip of homogeneous isotropic semiconduc'tive material and, by the application of a strong transverse electrostatic field, to control the resistance of the strip, and hence the current through it. I

So far as itknown, all of such past devices are beyond human skill to fabricate with the fineness necessary to produce amplification. In any event they do not appear to have been commercially successful.

"It is ,Well' known that in semiconductors there are two types of carriers of electricity which differ in the signs of the effective mobile charges. The negative carriers are excess electrons which are free to move, and are denoted by the term conduction electrons or simply electrons. The positive carriers are missing or defect electrons, and are denoted by the term holes. The conductivity of a semiconductor is called excess or defect, or N or P type, depending on whether the mobile charges normally present in excess in the r'iers) When a, metal electrode is placed in contact with a semiconductor and a potential difference is applied across the junction, the magnitude of the current which flows often depends on the sign as well as on the magnitude of the potential. A junction of this sort is called a rectifying contact. If the contact is made to an N-type semiconductor, the direction of easy current flow is that in which the semiconductor is negative with respect to the electrode. With a, P-type semiconductor, the direction of easy flow is that in which the semiconductor is positive. A similar rectifying contact exists at the boundary between two semiconductors of opposite conductivity types. V

This boundary may separate two semiconducto-r materials of different constitutions, or it may merely separate zones or regions, within a body of semiconductor material which is chemically and stoichiometrically uniform, which exhibit differen't conductivity characteristics.

body of the semiconductor.

The present invention in one form utilizes a block of semiconductor material on which three electrodes are placed. One of these, termed the trode and of that portion of the semiconductor which is in the immediate neighborhood of the electrode contact is such that a substantial frac tion of the current from this electrode is carried by charges whose signs are opposite to the signs of the mobilecharges normally in excess in the The collector is biased in; the reverse, or high resistance direction relative to the body of the semiconductor. In the absence of the emitter, the current to the collector fiows exclusively from the base elec-' trode and is impeded by the high resistance of this collector contact. The sign of the collector bias potential is such as to attract the carriers of opposite sign which come from the emitter. The collector is so disposed in relation to the emitter that a large fraction of the emitter current enters the collector. The fraction depends in part on the geometrical disposition of the electrodes and in part on the biaspotentials applied. As

, the matter is biased in the direction of easy flow." the emitter currentls sensitive to small changes than the change in the emitter current. The

collector circuit may contain a load of high impedance matched to the internal-impedance of the collector, which, because ot-the high "resistance rectifier contact of the collector, is high. As a result, voltage amplification, current amplification, and power amplification of the input signal are obtained.

one form, the device utilizes-a block of semiconductor material of whichthemain body'is of one conductivity type while a'very thin surface layer or film is of opposite conductivity type. The surface layer is separated from the bodyb'y high resistance rectifying barrier. The 1 emitter and {collector electrodes lmalie contact withthis surface layer sufliciently close together for mutual influence'in the manner described above. The base velectrode r naks 'a "law resistance contact w;ith :the body of the 'fsefilicdnductor. when suitable bias potntials'ar pplied'to the various electrodes, a, curre t fiow's ndia the emitter irito the thinlayer. owing to the conductivity of the layer and to the nature of the barrierfthis current tends to flow laterally in the thin layer, rathe rthan ifollowing'th'e most direct path across the barrier flto the base electrode. This current is xcom ppsed of carriers fwhos'e si'giis are opposite to the-si'g ns of the mobile lcharg' es normally in eiicess in the body of the semiconductor. In other ,words, whehtliere'is a thinlay'er'of opposite conductivity type "immediately under the e at r' eieetrofd itn feurrn flowing'into the in t'lie direction .crasy'flow consists largely 'iersldf 'oppds'itesign' to 'those or the 'r'ri'obile "s normally -jprisis'ntin excess the body block; anding-presence of these carriers i asesithelconddctivifiyfofthe block- The bias voltage 6p the collector which, as stated "above,

"biasfd in the reverse or high resistance dirction'rlatiye tof the block, "produces a strong electrostatic field 'in a "region "surrounding the cpllector 'sfd that tlie fcuirent from the'emitter whih efritrs this 'regionisdrawn'in to the collector. iiinusjtne pnectcr current, and hence the conductance "of lthe unit as "a .whble; are i creased. 'The ,ize of 'tHeY ifQnin which th s s. field exists is 'lomparati'velyf insensitive to var tions irfthe "collectorjpotential}so that the impedan'c'eof theoollectfoncircuit is high. on the other hafid, the current from the emitter to tlie layer is j tr'er'n'ely sensitive to variations of theenfiterip ential, so that the impedance ofthe emitt c rifuiti'is w. 1

t is a reanirepf theihvntion that the input and eaipteimraeaee or the "device ar controlled by choicejand treatment or the semiconductor -rnateri al -body and or its "surface, as well'asbyichoice-of the bias potentials of the electrodes. n I I From the standpbiilt of'i-ts external behavior and "usesfthe device er the 'inventionieseinbles awacuurn tube triode; andwhile'the electrodes are designated emitter. collector and base electrode;respective1y.-t er m bevlexiernelly interconnected in the various ways which have become recognized as appropriate for triodes, such as the conventional, the grounded gridfl the grounded plate" or cathode followe'r and thelike. Indeed, the discovery on which the invention is based was first made with circuit connections which are extremely similar to the so-called grounded grid" vacuum tube connections. However, the

analogies among the circuits are, of course, no

better than the analogy between emitter and cathode, Ibase electrode and grid, collector and anode.

'By'feeding back a portion of the output voltage in proper phase to the input terminals, the device may be caused to oscillate at a frequency determined byits external circuit elements, and,

among other tests, power amplification was confirmed by a, feedback connection which caused it to oscillate.

It has been found'that the performance .of the device .is "expressed, to a good approximation, .by the following functional relations:

where I e; emitter current It: collector current I e (W) :bollector current with emitter disconted Ve=voltage "of "emitter electrode measured with 'Ihe interprtatidn-of the foregoing Equation l-;i s-that the collector current lowers the potential' 'o'f the-surface of the blo'ck in the vicinity "of the' emititer"relativeito theibase electrode by an amount 'RFIc, -and thus increases the effective biasvoltageficn theiemitterby thei ame amount. The term "R1 11; thus representspositive feedback.

T invention w il' eruny comprehendedfrom the following detailed descriptionof one emb'o'dinienfi thereof, taken income-eco with the app nded drawings. in which:

R g 1 isaischematic dia ram, partly i perspective'showing a preferred embodimentof I the invention;

Fi 1a cross section or "a part of Fig. 1 t'o' a" greatly enlargedscale; 7

tag. an the equivalent vacuum tubesch'ematic circuitofFig. 1;

Fig. .3 is a plan view of .the block iof Fig. '1,

mg th'e disposition or the electrodes;

'F Ji3a' is like figfibutlshows theinfl'ue'nceof the collector,inimofdifyih the emitter current;

Figs. 4; 5, 6 and '57 i'showelectrode dispositions alternative'tothoselof-Figfil;

.igsfB-and'i9 slidwgelectrdde str'ucturesialternative to I those of -Fig. .1;

I0 shows amodifiediunit of the invention connected for operation inthe circuit of'a conventicnal'triode;

'Fig." 11 shows another modified .un'it 'ofthe' inver'ition onnected tor ,oper'atio in a "grounded plate or cathode follower circuit;

Fig. 12 shows the unit of the invention connected for self-sustained oscillation;

Fig. 13 is a diagram showing the electron potential distribution in the interior of an N-type semiconductor in contact with a metal;

Fig. 14 is a diagram showing the electron potential distribution in the interior of a P-type semiconductor in contact with a metal;

Fig. 15 is a diagram showing the electron potential distribution in the interior of a thin P-type semiconductive layer in contact on one side with a metal and on the other side with a body of N-type semiconducting material, for electrons in the conduction band (upper curves) and in the filled band (lower curves) and Fig. 16 is a diagram showing the variation of the potential distribution of curve b of Fig. 15 as a function of distance from the emitter to the collector.

The materials with which the invention deals are those semiconductors whose electrical characteristics are largely dependent on the inclusion therein of very small amounts of significantimpurities. The expression significant impurities is here used to denote those impurities which affect the electrical characteristics of the material such as its resistivity, photosensitivity, rectification, and the like, as distinguished from other impurities which have no apparent effect on these characteristics. The term impurities.' is intended to include intentionally added con stituents as well as any which may be included in the basic material as found in nature or as commerically available. Germanium is such a material which, along with some representative impurities, will furnish an illustrative example for explanation of the present invention. Silicon is another such material. In the case of semiconductors whichare chemical compounds such as cuprous oxide (CuzO) or silicon carbide (SiC').

deviations from stoichiometric composition may constitute significant impurities.

Small amounts, i. e., up to 0.1 per cent of impurities, generally of higher valency than the basic semiconductor material, e. g., phosphorous inlsilicon, antimony and arsenic in germanium, are termed donor impurities because they contribute to the conductivity of the basic material by donating electrons to an unfilled conduction energy band in the basic material. In such case the donated negative electrons constitute the carriers of current and the material and its conductivity are said to be of the N-type. Similar small amounts of impurities, generally of lower, valency than the basic material, e. g., boron in silicon or aluminum in germanium, are termed acceptor impurities because they contribute to the conductivity by accepting electrons from the atoms of the basic material in the filled band. Such an acceptance leaves a gap or hole in the filled band. By interchange of the borrowed electrons from atom to atom, these positive holes effectively move about and constitute the carriers of current, and the material and its conductivity are said to be ofthe P-type.

Under equilibrium conditions, the conductivity ofan electrically neutral region or zone of such a semiconductor material is directly related to the concentration of significant impurities. Donor impurities which have given up electrons to an unfilled band are positively charged, and may be thought of as fixed positive ions. Ina region of a semiconductor which has only donor typeimpurities, the: concentration of conducion l mnsiseque1 to. th concentration f ionized donors. Similarly, in a region of a semiconductor which has only acceptor impurities,

the concentration of holes is equal to the concentration of the negatively charged acceptor ions.

5 If for any reason there is a departure from electrical neutrality in a region, giving a resultant space charge, the magnitude of the conductivity, and even the conductivity type may difier from that indicated by the significant impurities. It was once thought that the high resistance barrier layer in a rectifier differs somehow in chemical constitution or in the nature of the significant impurities from the main body of the semiconductor. W. Schottky, in Zeits. f.

Phys, volume 113, page 367 (1939), has shown that this is not necessary. While the concentration of carriers (mobile charges) in the barrier layer is small, the concentration of ionized impurities (fixed charges) may be the same as in the body of the semiconductor. charges in the barrier layer act in concert with induced charges of opposite sign on the'metal electrode to produce a potential drop between the electrode and the body of the semiconductor..

The concentration of carriers at a point depends on the electrostatic potential at that point, and is small compared with the equilibrium concentrae tion in the body of the semiconductor if the potential differs from that in the body by more than a small fraction of a volt. The methematical theory has been developed by W. Schottky and: E. 'Spenke in Wiss. Veroif, Siemens Werke, Vol.18,

page 225 (1939). These authors show that if the variation in electrostatic potential with depth betivity passes through a minimum for a certain potential and depth and the conductivity is of opposite type for larger values of the potential corresponding to smaller values of depth. They ing contacts may exist beneath the free surfaceof'a semiconductor, the space charge of the barrier layer being balanced by a charge of opposite sign on the surface atoms. It is possible, for 'ex-,

ample, to have a thin layer of P-type conductivity at the free surface of a block which has a uniform the block, even though there are no actual acceptor impurities. 6 To distinguish such a situation from the similar one which depends on the presence of significant chemical impurities of opposite type in a thin surface layer, the terms physical" and chemical are employed. Thus, the terms the layer of opposite conductivity type next to the surface and the high resistance barrier which. separates it from the body of the semiconductor,

both of which exist as a result of surface conditions and not as a result of a variation in the nature or concentration of significant impurities. I The terms chemical layer and chemical bar rier, refer to the corresponding situationwhich does depend on a variation in significant impurilow the surface is sufiiciently large, the conducconcentration of donor impurities and which therefore, has N -type conductivity in the body of 1 physical layer and physical barrier refer to" Both physical layers and chemical layers Hare suitable for the invention.

Its-is known how, by-control of the distribution of impurities, to fabricate a block of silicon :of which the main body is of one conductivity type while a thin surface layer, separated from the main body by ahigh resistance barrier, is foflthe other type. In this case the layer I is believed to be chemical rather than physical. For-methods of'preparing such silicon, as well'a's for-certain uses of the same, reference may be made to an application of J. :H. Scaif:and H. C. Theueren'fil'ed December 24,l94'7,Seri-al No. 793,744, nowTPaten't No. 2,567,970, issued September 18, 1951, and "to United States Patents 2,402,661 and 2,402,662 to R. "S. Ohl. Such materials are suitable forusein connection with 'the'present invention. .It is'preferredjhowever, todescribe the invention in connection with the material which was employed when the discovery on which the i'nventionis based was made, namely, N-type germanium which has been so treatedas to enable'itto withstand high voltage in the reverse direction when used-as 'a point contact rectifier.

There are a number of methods by'whic'h the germanium and its surface may be prepared. Gne such method commences with the process which forms the subject-matter of an .applica-' tion'of J-. H. Scarf and l-L'C. Theuerenfileii December 29,' 19'45, 'Serial :No. 638,351, and which is further described in Crystal Rectifiers," by H. C. .Torrey and C. A. Whitmer, .RadiationLaboratory Series No. 15 (McGraw-Hill, 1948). Briefly, germanium dioxide is placed ina porcelain dish and reduced togermanium in a furnace in:an atmosphere of hydrogen. After apreliminary low heat,. the temperature is raised to 1,000 C'. at which the germanium is liquefied and substantially complete reduction takes place. Thecharge is then rapidly cooled to room temperature,

whereupon it may be broken int pieces of convenient size for the next step. Ihe charge is now placed-in a graphite crucible and heated to liquefaction in an induction furnace in an atmosphere of helium and then slowly cooled from the bottom upwardly by raising the .heating'coil at the rate of about inch per-minute until the chargehas fully solidified. It is then cooled to room temperature.

eingot isnext soakedat a .lowheat ofabout 500 C.Jfor .24 (hours in a neutral atmosphere, for example, of helium .after which it is allowed to cool to room temperature. I

In the resulting heat-treated ingot, various parts orzonesare of various characteristics. In particular, the central part of the ingot is of N-g type material capable of withstanding "a *back voltage, in the sense "in which this 1 term is employed in the rectifier art, of.100-2'G 0*vo'lts. It-is this material which it is preferred to --employ in connection with the present invention. 7

Thisinaterial is'next cut into blocks of suitable size and-shape for use in connection with the invention. A suitable shape "is 'a disc "shaped bloch ofabout Aymeh diameter, and inchthickness. The block is-then groun'd'fla-t on b'oth sides, first with 280 mesh abrasive dust, for -example, -C'ar- I borun'dum,'and then with 600 nresh. It is then etched for one minute. The etching =solutionmay consist of 10c. c. of concentrated -ni-tric-acidj 13.13. V of commercial standard 150 per cent) hydrofluoric'acidyand c. c.of 'water'in'w'hich asmall amount. e. g. 0f2 -gram,of copper =nitrate' has been dissolved. This etching treatment enables the block to withstand high (rectifier) back voltages.

Next-one side :of the block is provide'cl'with a coating of metal, for example copper or gold, which constitutes .a low resistance electric contact. This may be done 'by evaporation -or electroplating in accordance with well-known techniques. As a precaution against contamination of the other (unpl-a'tedt) side of the block which may have -'occurred in "the course of the plating process, the unplated side may be subjected to a repetition of the etching process.

The block may now be given-an anodic oxidation treatment, which maybe carri'ed ou't in the following way. The block is placed, plated side down, on a metal bed-plate which is connected to the positive {terminal of asour-ce of voltage such as a battery, and that part of the upper-(unplated) surface which is to be treated is covered with polymerized glycol bori'borate, or otheripreferably viscous electrolyte in which germanium dioxide is insoluble. electrode of inert-metal, such as silver, is dipped "into the liquid without touching the surface of the block, and is con-- nected to a negative "battery terminal of about "22:5 volts. Current of about 1 m'illiampere commences to flow "for each square centimeter of "block surface, falling to about "0:2 milliampere per cmi in about '4 minutes. The electrode is then connected to the volt battery terminal. The initial current is about 0.7 milliampere per crniZ, falling to 0:2 milliampere'per cm? inabout The electrode is then connected to 'be evaporated onin the course of the finish-drying process. .The;germanium block iis now ready foruse.

The foregoing oxidation :process, however, is not essential. Amplification has been obtained with specimens to which no surface treatment has been applied subsequent to the etch, other than :the electrical iforming process described below.

fFig. ls'hows =a block -l of germanium which has been prepared .in the ioregoing manner, and Fig.

1a shows the :central part of theblock l insection .and :to anenlargedsca-le. Referring to "Figs. land in together, the lower part of the block '1,

.whose surface is plated with a metal film 2 servis well knowmthe boundary 4 separating-this P- type layer from the N- typemateri-alof the main body *of the block behaves like a high resistance rectifying barrier. A first electrode 5, denoted the emitter, makes contact with the 'upper face of the block, i. 'e., with the P-type -layer 3, pref erably somewhere near itscenter, orat least sev- I'from 0 .5 to 5 mils diameter, preferably pointed by way of an input transformer l0.

9 at the contact end electrolytically or by grinding. Processes for forming the points on such wires are described in United States Patent. 2,430,028 a to W. G. Pfann, J. H. Scaff, and A. H. White. The point of the Wire is brought into contact with the upper surface 3 of the block with a force of l'to grams, whereupon a cold flow of the metal of the point takes place, causing it to conform to any minute irregularities of the block surface. To this end the wire of the point should be ductile as compared with the material of the block. Tungsten, copper and Phosphor bronze are examples of suitable materials.

'A second electrode 6, denoted the collector, makes contact with the upper face 3 of the block very close to the emitter 5. Best results have been obtained when the separation, measured along the surface of the block, between the collector and the emitter, is from 1 to 10 mils. This electrode 6 should make rectifier contact with the block and may be a pointed spring wire, formed and placed as above described in connection with the emitter 5. On the other hand, it may comprise a small spot of metal, for example, gold, which has been evaporated onto the upper surface of the block in the course of the final drying operation, and through which a central hole has been pierced (see Fig. '6) or across which a diametral slot has been out (see Fig. 7). Evaporation of such a spot or film of metal onto the upper face after completion of the anodic oxidation process described above results in anonohmic rectifier junction or connection.

A third connection, termed the base electrode,

is made, by soldering or otherwise, to the metal film 2 which has been plated onto the lower surface of the block I. While the unit is now ready for use, its operation can generally be improved by an electrical forming process, in which a potentialin excess ofthe'peak back voltage is applied to either one or both of the point electrodes 5, 6, i. e., between itand the base electrode 2. The unit is protected from injury by heavy currents by inclusion of a resistor in series. The effect of this treatment is believed to lie in a concentrated application of electric field and heat, to the material in the immediate neighborhood of the point, and so in an improvement of the electrical characteristics of the contact.

Bias voltages are now applied to the electrodes, a "small bias, usually positive, on theemitter of the order of a fraction of a volt andla larger negative bias on the collector, usually in the range from 5 to volts, in each'case, from thebody of the block to the point electrode.

These bias potentials may be obtainedfrom batteries 1, 8 connected as shown or otherwise, as desired. a

A load of 1,000 to 100,000 ohms may now be connected in circuit with the collector, for example by way of an output transformer 9, and a signal to be amplified may be applied between the emitter and the base electrode, for example The connections may be those of the conventional triode as indicated in Fig. 10, or of the so-called grounded plate or cathode-follower, as in Fig. 11. In. these figures the input signal is symbolically represented by a source II and the load by an output resistor Rh. Discovery of the amplifying properties of the device was made, however, with the grounded base circuit of Fig. 1, of which the vacuum tube counterpart is the so-called grounded grid connection of Fig. 2. (The principal distinguishing feature of this circuit as employed with a vacuum tube triode is that the load current flows through the source. This does not hold for the unit of the present invention, because the base electrode may draw substantial current.) Th device as thus connected has given power gains of more than a factor of 75. Operating data on three different samples are given in the following table:

Sample No 1 2 3 I Input Res. (ohms) 640 500 1000 Output Res. (ohms) 3X10 3X10 3X10 Input Voltage A. C. R. M. S. 0.29 0.30 0. 10 Output VoltageA. O. R. M. S1. l8 l5 3. 6 Voltage Gain 62 50 36 Power in (watts) 1. 3X10- 1.8X10' 1.15Xl0- Power out (watts) 100x10 75Xl0- 42. 5X10- Power Gain 42 36 Input Bias D. 0. (volts) +0.2 +0.25 +0. 2 Output Bias D. 0. (volts)... 40 -20 -10 Confirmation of the presence of power amplification has been obtained by feeding back a part of the output voltage to the input circuit, as by way of the coupling between the windings of a transformer I2, as in Fig. 12 whereupon sustained self-oscillation took place.

It is to be noted that in the case of the No. 1 sample of the foregoing table, the power gain exceeds the voltage gain by a factor of' Inasmuch as, in any amplifying device which gives both power gain and voltage gain, the current gain is the quotient of the two, it is evident that sample No. 1 manifests a current gain of 1.3.

Without necessarily subscribing to any particular theory, the following hypothesis is presented to account for the experimentally determined facts, with all of which it is consistent. It is believed that the preparation of the semiconductor material and its surface treatment result in the formation of an oxide film, and, below it, of a layer or film 3 of P-type conductivity on the surface of the block, separated from the main body, which is of N-type, by a high resistance barrier 4. The oxide film is removed by washing. "This P-type layer is very thin, perhaps 10- cm. in thickness, but the N-type body of the block provides all necessary support for it, and also provides a low impedance path tothe base electrode 2. Its presence is confirmedby the'fact that, particularly with featherweight forces on the contact points 5, 6 and with small voltages applied to them, P-type rectifiercharacteristics have sometimes been obtained. (P- type and N-type rectifier characteristics and their significance and. differences are discussed in United States Patent 2,402,839 to R. S. Ohl.) But When the mechanicalforce on the contact point is increased to 10 grams or so and the voltage applied to it is raised to /2 volt or so, the rectifier characteristic is observed suddenly to shift from P-type to N-type. Furthermore, potential probe measurements on the surface of the block, made with the collector disconnected, indicate that the major part of the emitter current travels on or close to the surface of the block, substantially laterally in all directions away from the emitter 5 before crossing the barrier 4. These measurements indicate the presence of a conducting layer at the surface of the block, which by inference is. of P.-'-type1. In case:- the treatment stops. with i the; etching process; the layer: is believed to: be

physical. If it includes. the further anodicoxidation-stem the: layer is believed tobe, chemical,

.but itssnature has noti-beendefinitely established. It is, believedthat the P-type layer. on. the

germanium surfaceof the'preferredembodiment is not. -grea-tly altered when a contact is made with ametal point. When a small-positive bias is applied to the emitter, and a current-flows, the carriers are largely those of the surface layer, that is, holes rather than conduction electrons. The potential probe measurements discussed above indicate that. the concentration ofv holes, and thus the conductivity=,. in. thevicinit-y; otthe emitter point, increase with increasing forward current- This hole,currentspreadsout. in all} directionsz from the. emitter before; crossing the high resistance barrier Withthe. collector circuit open, it then makes its Way throughout: the body of the :block to the plated lower surface 2. (In the N-type body of the block, the current may take the term, of a flow of electrons upward to neutralize the downward flow of holes fromthe P-typelayer.) In the absence of the collector electrode 6, this-current is the only current. Its path is indicated in Fig; 1a bystreamlinesit.

Now when the collector 6. contact is made, and a negative bias potential is applied to it, of from -5-to 50 volts, a strong electrostaticjfield: appears across the P-type layer 3, and across the high resistance barrier l, being maintained by the fixed positive charges in the N-type body material in the immediate vicinity of thecollector. The barrier and the P-type layer together are believed to. be of the. order. of: cm. in thickness. Thus, with IO volts. across a space of 10" cms., the aVeragestrength of this field 15,01? the order of 10 volts,per cm., being greatest at thecollector and extending, in all directions from the collector, and is indicated in Fig. la by the broken line 14', within which some of the fixed positive charges are indicatedirby' plus signs.

It is in order that the material shall be. able to support alarge voltage drop. across this region that; material of the so-called high back voltage type is pref erred;

Now when the'current of positive holes as in. dicated by stream lines l5 comes. within the influence of this field, the holes are attracted to the region of lowest: potential namely, tothe point at which the collector electrode 6' makes contact with the layer 3'. There they are picked up by the collector 6' to appear as. a current in an external load circuit 8, 9 connected to the collector 6. With the large negative bias on the collector 6, a variation of several volts on the collector makes very little diiference inthe strength or the extent of the field which surrounds it, and therefore hasbut a secondaryefrect on the fraction of the emitter current collected by' the collector. In other words the collector operates: under conditions which are close to saturation, and the alternatin current impedance of the collector circuit is high. As shown in above table, it has been measured at values from 10,000 to 100,000 ohms. For maximum power, output,.. the external load impedance should. be. matched to the internal impedance of the: collector; On the: other hand, variation of the: voltage. betweenthe-emitter 5 and the. base electrode 2 by a small fraction of avoit', as by a: signal which may-be; applied to the input terminals' and so impressed on these. electrodes, for example; by way-of the transformer), eiiects a large: variation]. in theemitter current and there;

.flowing. laterally in thesurface layer itself; 1. e.,

the spreading resistance-of the layer.

When the collector electrode 6.- is' a. single pointedwireor an evaporated. metal spot, a fraction;ofthe emitter current, after spreading out laterally inthe P-type layer. 3., eventually finds itsway. across the barrier to the plated elec trade 2 onthelowerface. oi the.- block, i. e., to the base. electrode. This situation. isdepicted. in Fig. 3. which is. a plan view of the block. showin current stream lines. I3. diverging. in all. directionsfrom theemitt'er.v Thecurrentstream lines Iiiare, straight in the. absence of the. collector field. Whenthe. collector field l4 ispresentthe currentfieldis distorted .asinFig, 3a. whichshows that even with a single collector electrodefi more thanhalf. of the. emitter currentcan be. collected. Intact, theiractioir of the. emitter. current which reaches the. collector may in favorable. cases be ashighas 9.0. per cent.

To increase this ratio. especially in thecase of units in whichthis ratiois. less.favorable,.requires a modifiedelectrode arrangement. Obviously, if the strong. field; l li surrounding. the, collector B were, to. overlap or include the. emitter. 5,. sub.- stantially. all ofthe emitter current would be cc!- leeted. This, however, would involve a. loss of control. A solution. is. to. provide-twocollectors 6. 6a, as in Fig, 4;.or. three 6.. 6a., 6b, as in. Fig. 5,

symmetrically disposed. about the. emitter 5.

lector field l4. bearsthe shape ofasemitorus. Its trace on the plane of the block surtfaceisshown by the broken lines I4'a, [4b. The two. semicircular spots. 66, (if, of Fig. 7. are the substantial equivalent or the circle of Fig.6.

Further increasemay be made in the effective resistance of the barrier 4. and. therefore. in. the internal resistanceof the, emitter-base electrode circuit and of the ratio of the collector current to the emitter current, by'restricting the. area of the barrier 4 itself to a comparatively small reion surrounding the emitter 5 and the collector 6. This may beaccomplished by restricting the area of the block lwhich is subjected to the anodic oxidation treatment or by machining the block after treatment. In the former case the result is a bowl-shaped P-layer- 3', bounded by a bowl-shaped barrier 4 as shown in Fig. 11, and

ihthe latter case it is a block I having the form a. truncated pyramid, with the barrier 4" close to the; smallest face, as indicated in Fig. 10'.

In the event that the spring. feature is not de-" sired for the: emitter and: collector contact points. various alternative structures. may be employed. For; example, two sides of a: wedge-shaped piece of insulating. material: It may be plated with metal films as in Fig. 8, one 5| to serve as emitter andlthe other 6| as collector. Ora cone-shaped piece I! may be plated over its conical surface and awire inserted through a central hole as in Fig. 9-. The central wire 52 is preferably employed as the emitter and the surrounding plate film 62 as collector. The cone and the wedge serve to hold the interelectrode capacities to a minimum while keeping the contacts close together where they bear against the surface of the semiconductor.

Further understanding of the considerations which govern the thickness of the P-type surface layer may be had from the following considerations, which apply. specifically to a chemical layer. Fig. 13 is a plot of the electrostatic potential within the body of an N-type semiconductor in contact with a metal. As above stated, the N- .type material of the semiconductor contains fixed or. bound positive charges. They are believed to be distributed with fair uniformity in depth to a certain distance, beyond which the material is electrically neutral, because the bound positive charges are balanced by equal negative (electron) charges. In accordance with Poissons equation:

2. m- 6 where V is the potential is: is the distance, measured from the metal into "the semiconductor p is the charge density, and e is the dielectric constant of the material.

Assuming the charge density p to be uniform,

two integrations give the potential as a function ofdepth. When plotted, it is a parabola. In the figure, negative potential has been plotted up-' ward. The vertical rise Ee from the Fermi level to the terminus of the curve, 1. e., to its intercept,

with the potential axis, represents the energy .which must be imparted to an electron to cause it tomove from the metal to the semi-conductor. These matters are fully explained in the literature, for example, in Schottkys theories of dry 14 each group represent the conditions when a signal applied between the emitter and the control electrode further reduces the potential of the block. Evidently the alteration of the block potential with respect to the emitter operates in each case to increase the effective thickness of the P-type layer and so the density of holes and the layer conductivity. Such an increase in consolid rectifiers, by J. Jofie, published in Elec- Fig. 15 is a composite diagram showing, in the upper curves, the electron energy and in the lower curves the hole energy, within a semiconductor which comprises a thin layer of P-type material separated from a body of N-type material by a.

barrier. The fixed charged are negative in the Pr-type material and positive in the N-type, and

for simplicity are assumed to be distributed uniformly in each zone. Integration of the charge density, twice,in accordance with Poissons equa: tion gives the lowermost curves, a, b of the two groups, which represent equilibrium conditions and which, but for an additive constant E are alike. The constant E; represents the energy difference between the filled band and the conduction band for the particular material.

The middle curves or, D1, of each group represent the conditions when a small negative bias is applied to the semiconductor block I with respect to the emitter 5, and the upper curves (:2, D2, of f ductivity with increase in the forward bias has been observed in connection with the potential probe measurements referred to above.

The rounded peak of the hole potential curve lies below the Fermi level. The greater the thickness of the P-type layer, the more the terminus of this curve falls below the Fermi level, i. e., the

greater the magnitude of Eh, and the greate the.

holes move from the metal of the emitter to the semiconductor and enter it. On the other hand, if the P-type layer is too thin, the conductivity of the layer, which is related to the width of the approximately fiat portion of the upper part of the curve In of Fig. 15 will be small. In the vicinity of the collector electrode, the thickness of the 'P-ty-pe layer should be sufficiently small so that the rectification characteristic of the collector is determined primarily by the body of the semiconductor and not by the layer. If, now, the collector is biased in the reverse direction relative to the body, most of the drop from the high voltage on the electrode occurs in the immediate vicinity of the collector, so that the impedance of the collector circuit is high.

The P-type layer is preferably adjusted to an optimum thickness lying between these extremes. Best results are believed to be obtained when its thickness is such that the terminus of the curve falls slightly below the rounded peak. Holes can enter the semi-conductor without great difficulty, and tend to collect in the region of greatest negative potential as a cloud of mobile positive charges. They then diffuse away from the emitter-laterally in Fig. 1, perpendicular to the paper in Fig. -15,--some of them entering the field M of the collector B.

Because the right-hand part of the lower curve falls well below the left-hand part, positive holes can cross the barrier only with difiioulty. Because the P-type layer is thin, the energy Eh required to cause holes to enter the layer, is small.

Therefore, holes enter easily under the influence C :m e1n1+ 112621122 (3 where m, e1, ,ul are the electron density, the electronic charge, and the eleotrons mobility, respectively, and a m, 62, 1.2 are the corresponding quantities for ns i h l s. 7

' bewritten Vathe, heightcf. the electron space potential curve: (a of Fig; 15) above, thesFermilevel, andVh is, correspondingly, the heightof. the Fermi level above thehole space potential. curve (1) of Fig. l5) and.A-1, A2, K, a-nd T are constants for a given. temperature. Inasmuch as, the potential difference.between-the two. kinds of space potentiaLcurves isa constant E the conductivity may 1#i r KT+A2IL26T6 (5) Since. the'factor A er. does, not diiier, greatly in magnitude. from the factor Azpzez it a simple matter of. calculation, to show that this expressionis a-minimum when.

i: e., that the resistivity of the material is greatest atthe. depth. at which the a: curves and. the. b curvesofFi-g. l5 lie at equal distancesabove and beiow theLEermi level, respectively; and. that. furthermore, the resistivity departs rapidly. from this maximum value as the space potentials Ve and Vh depart-from equality. If

the electron conductivity is greater than the hole conductivity, and the conductivity is N-type. If

the-hole conductivity is greater than the-electron conductivity, and the conductivity i-s P-type.

Fig-16s is a three dimensional representation of" the conditions which the holes-- encounter in the course: of their travel in the layer from the emitter tothe collectorin the figure, parallel with the Y axis. As in Fig. 15, theX axis-represents depth measured into the semiconductor and the V axiswhich is drawn to an approximately logarithmic scale, represents negative potential. As. the=holes approach the collect-or the peak of the. potential curve becomes less and less prountil. finally, at thecollector, the region lowest: potential, to. which the holes flow, is the collector itself, where they arewithdra'wn.

- Of. that part of the emitter current". which crosses the barrier, a certain fraction crosses it againfin the: vicinity of the collector and is collected, thus forming a part of the collector currenti. Theforegoing hypothesis as to the mechanisnt by which. amplification is obtained applies to this: fraction. of. the current as well as. to. the fraction which. proceeds entirely within the layer.

The: collector current contains still another component, which consists of a flow of electrons from the collector. tothe base electrode, crossing the barrier once on its way. A hypothesis as to the manner in which this current component takes part in the amplification process is as follows:

There is a. potential hill at the contact point between the collector electrode and the surface layer which offers an impedance to the flow of electrons from the electrode into the semiconductcr. In: theiabenceof'bias, the height of this hill. indicated by. Es. Figs. 13; and. 1'5, is the en.- ergy required; to take an. electron-from the metal and placeitzirr theconduction band: of'the semiconductor. Whom the: collector is biased in. the reverseidirectlom the effective height of the hill; and sothe impedance of; the contact points, are reduced: by the: electric field across the layer and barrier which act in such a direction as topu'll electrons from the electrode. The efEect is to increase. the how of electrons. into. the: semiconductor: in a way. which. is. simil'artov the" enhancement; ofv current from. a thermionic cathode by field-induced emission. When theemitter is con.- nected; and-avv current. of holes flows to the col:- lector; the accumulation of the positive charges of; these holes inthevicinity of the-collector tends to: make: the potentiaL fall more rapidly with depth into the material,. and. so results in an in.- crease: in field. and. adecrease. in. the. effective height of: the hill,,. i-.. e., in. the impedance of" the contact point; Thus: any: increase: inthat com:- ponent of the collector current which originates at the; emitter is: accompanied by acorresponding increase in the other component of the collector current, namely, in the nowof electrons to the base electrode. Hence, the total change in collector current may be greater than the change in the emitter current. Y 4

From. the foregoing. description it. will be clear that if it is desired to employ a semiconductor block of which the main bodyis-oi P-type so that the conductivity oi the thin. surface layer, whether due to impurities or to space charge eliects, is of N-type, the polarities ofall the bias sources of Figs. 1,10, 11 a-nd=1'2 areto' be reversed. It is also to be understood that the magnitudes of the-biases for best operation will depend onthe semiconductor-material' employed and on its heat treatment and processing. Furthermore, it is possible to use a P'-type.- layer' of one semiconductor-material on an N-type body of some other semiconductor material or; vice versa. All such Variations are contemplated as'being' within the spirit of the invention.

The inventionis' not to. be construed as limited to the particular forms disclosed herein, since. these are to be regarded as illustrative rather than restrictive.

What is claimed is:

I. A circuit element which comprises a body of semiconductor material, a block of: insulator material haying surfaces which are angularly disposed with respect to. each other, juxtaposed with said body, and an electrode fixed to each of said surfaces; the ends of said electrodes whose mutual spacing is leastmaking contact with said body, oneof said electrodes being disposed to collectcurrent spreading in said body from the other of said electrodes.

2. A circuit element which comprises a supporting body, a thin surface layer of semiconductormaterial supported by said body and differingin conductivity therefrom, a block of insulator material having the form of a wedge, truncated near to its apex, juxtaposed with said layer; an electrode fixed to eachof the angul'arly separated faces of the wedge, the ends of said electrodes which arenearest tothe truncated and of the wedgemaking contact with said layer, oneof said electrodes being disposed to collect current spreading in said layer from the other of said electrodes, and a base electrode providing a low resistance connection to' said body to influence the magnitude of said spreading current.

A circuit element which comprises a. supporting body, a thin surface layer of semiconductor material supported by said body and differing in conductivity therefrom, a block of insulator material having the form of a cone, truncated near to its apex, juxtaposed with said layer, a first filamentaryelectrode axially threading said cone, a second film electrode fixed to the conical surface of said cone, the ends of saidelectrodes which are nearest to the truncated end of the cone making contact with said layer, respectively in a point and in a closed curve surrounding said point,,one of said electrodes being disposed to collect current spreading in said layer from the other of said electrodes, and a base electrode providing a low resistance connection to said body to influencethemagnitude of said spreading current. I

4. A circuit element which comprises a body of semiconductor material, aablock of i insulator material having the form of a wedge, truncated near to its apex, juxtaposed with said body, and an electrode fixed to each of the angularly separated faces of the wedge, the ends of said electrodes which are nearest to the truncated end of the wedge making contact with said body, one of said electrodes being disposed to collect current spreading in said body from the other of said electrodes.

5. A circuit element which comprises a supporting body, a thin surface layer of semiconductor material supported by said body and differing in conductivity therefrom, a block of insulator material having oppositely located faces juxtaposed with said layer, an electrode fixed to each of said faces, an end of each of said electrodes making contact with said layer, one of said electrodes being disposed to collect current spreading in said layer from the other of said electrodes, and a base electrode providing a low resistance connection to said body to influence the magnitude of said spreading current.

6. A circuit element which comprises a supporting body, a, thin surface layer of semiconductor material supported by said body and differing in conductivity therefrom, a block of insulator material having at least two faces which are angularly disposed with respect to each other, juxtaposed with said layer, an electrode fixed to each of the said faces, the ends of said electrodes whose mutual spacing is least making contact with said layer, one of said electrodes being disposed to collect current spreading in said layer from the other of said electrodes, and a base electrode providing a low resistance connection to said body to influence the magnitude of said spreading current.

7. A circuit element which comprises a body of semiconductor material, a block of insulator material having the form of a cone, truncated near to its apex, juxtaposed with said body, a first electrode axially threading said cone, and a second film electrode fixed to the conical surface of said cone, the ends of said electrodes which are nearest to the truncated end of the cone making contact with said body respectively in a point and in a closed curve surrounding said point, one of said electrodes being disposed to collect current spreading in said body from the other of said electrodes.

8. A circuit element which comprises a body of semiconductor material, a tapered block of insulator material juxtaposed with said body, a first filamentary electrode axially threading said asp-sea 18 covering the external surface of said block, the ends of said electrodes which are nearest to the narrow end of the tapered block making. contact with said body respectively in point and in a line surrounding said point, one of said electrodes being disposed to collect current spreading in said body from the other of said electrodes.

9. A circuit element which comprises a body of semiconductor material, a tapered block of in sulator material juxtaposed with said body, a first filamentary electrode axially threading said block, and a second electrode fixed to the external surface of said block, the ends of said electrodes which are nearest to the narrow end of the block making contact with said body, one of said electrodes being disposed to collect current spreading in said body from the other of said electrodes.

10. Acircuit element which comprises a body of semiconductor material, a blockof insulator material having surfaces which are angularly disposed with respect to each other, juxtaposed with said body, and an electrode fixed to each of said surfaces, the ends of said electrodes whose mutual spacing is least making contact with said body, one of said electrodes being disposed to collect current spreading in said body from the other of said electrodes, and a base electrode making contact with the body.

11. A circuit element comprising a body of semiconductor material, one portion of which is of one conductivity type and another portion of which is of different conductivity type, a block of insulator material having surfaces which are angularly disposed with respect to each other, juxtaposed with said body, an emitter electrode fixed to one of said surfaces and engaging the first portion of the body, a collector electrode fixed to another of said surfaces and engaging the body to collect current flowing to the body by Way of said emitter electrode, and a base electrode providing a low resistance connection to said other portion of thebody to vary the magnitude of said current.

12. A circuit element comprising a body of semiconductor material, one portion of which is of one conductivity type and another portion of which is of different conductivity type, a block of insulator material having surfaces which are angularly disposed with respect to each other, juxtaposed with said body, and an electrode fixed to each of said surfaces, the ends of said electrodes whose mutual spacing is least making contact with said body, a first one of said electrodes being an emitter electrode and engaging said one portion of said body, a second one of said electrodes being a collector electrode and engaging the body to collect current flowing to the body by way of said emitter electrode, and a base electrode providing a low resistance connection to said other portion of the body to vary the magnitude of said current.

13. A circuit element which comprises a body of semiconductor material, one portion of which is of one conductivity type and another portion of which is of different conductivity type, a block of insulator material having the form of a wedge, truncated near to its apex, juxtaposed with said body, an electrode fixed, to each of the angularly separated faces of said wedge, the ends of said electrodes which are nearest to the truncated end of the wedge making contact with said body, one

of said electrodes being an emitter electrode and engaging the first portion of the body, another of said electrodes being a collector electrode and engaging the body to collect current flowing to the 19 bcdy by wayof saideiniiiter electrcde, and a base electrode prci idin'g lcw resistance ccnnectio'n'to sa'iicfihef DOr'tioii'ci the body to vary the magnitude pr said current. 4

'14. A circuit; element co 'iii'ris-ing a body of semiconductor materiel, ne iiqrtioxi of which is of one con'diic't'i'vity ty e andencther "pcr'tior of which is of: difierentccr ductivity type, a bleak Qf irisuleto materiel heviiig'the form (if a eerie BTUQGQEQQ pea;- "to its "apex, juxtaposed with said bogly, a fi isi; filamentary electrode axially threadief ee q w mE idflm e c rode fl w the cc' 'iica 'l sfi fage c'i" said-gone, ial'ieende of said eleco s Whifl eeres fiq'ih t nc nd of H g ge tget "with "saifi body respecp ie jen 'elei l i. c e u o i i i en "o e QI- kiiq ele od s being an 'eiriiiir i e dd 'en n n e fir p rt e of Eh? bq ywi h? 9 id 'e i de e n M lq iql' P'Q Q a d en a n the bb'dy i e ll mft e eowfe to. ih 'bo b wa rs idtt r e e' de an e baee1e r e ovid i a 10 if'e i 'fieeeei le fiiqe 9 O h r P rt f. t ee ary he ma mmqeq S ur 55 e'e q element wh ch q mnri es a e y semiqqnq iq'iieg were. a fi teleqtrq mak n contact with seid body over a relatively large area, and at least two other electrodes each of which makes'contact with saidbody over a narrow REFERENCES CITED The fcil'cwipg references age of record in the file of this patent:

TAT S BA-TEN Number Name Date 21438393 Bieling s- Apr. 6, 1948 2,459,787 Bloom Jan. 25 1949 2,486,776 Barney 1 Nov. 1, 1949 2,503,837 Ohi Q. Apr. 1 1, 1950 2,5 60392 Gibney Jul 17-, 1951 

